Keep a room description short
My players, which I think are fairly average, tend to tune out after the second or third line of a room description, so it's critical to get right to the point with the most important information. If I go on at any length, they're going to stop listening. And once they tune out, it takes extra effort to pull them back into the game.
The strategy I use is basically three sentences: First give a broad overview of the room (or area) for general scene-setting; then give it flavor with a sensory experience; and finally describe one or two points of interest in the room. The important information should come in part 1 and part 3 -- the middle is the most likely bit to get ignored, so it's a good place for some mood-setting description that doesn't actually do anything critical.
In a lot of older modules, I see a particular style of describing rooms that kind of drives me nuts. It'll say something like: "This chamber is twenty feet square and ten feet high. The floor of the chamber is dressed stone. The walls are white marble with blue inlaid tiles. Against the back wall there is a large bed draped in velvet with a trunk at its foot. Tapestries and bookcases full of ancient, dusty tomes line the walls. A simple, sturdy oak table in the center of the room holds an ornately carved ivory statuette of a dolphin."
Okay, that is marginally serviceable description of the room, but how much of that did the players actually care about? Were we particularly invested in the exact room size or the flooring that got top billing? By the time we've mentioned the tapestries and the statuette, most of my players have gotten distracted, or have focused on the velvet bed and didn't even notice me mention the bookcases. Worse, I feel like this kind of mechanical description doesn't paint a mental image of the room in any meaningful way.
I'd say something like:
"You find a bed-chamber that is surprisingly plush compared to the tunnels outside, walls lined with bookshelves and tapestries. It smells of sweet herbs and beeswax candles. A velvet-draped four-poster bed dominates one wall, and in the center of the room is a heavy table holding a statuette."
But don't make it a guessing game
Notice how much I didn't say about the room. I didn't discuss what the statuette looks like or is made of. I didn't mention what's on the shelves. Those are details I'll fill in as the players start moving into the room and looking around.
There can be a fine line between making your room description brief and turning the room into a guessing game. But there is, I think, a good reason to do this.
People tend to only be able to hold a few pieces of new information in their head at once. Mentioning too many things at once very quickly drives out the first pieces of information without getting really processed, or the player fixates on early information and misses the later parts. This often manifests as catching the general gist of the description but apparently missing important specifics. By saying "there's a statuette" and moving on, for example, the players don't get the extra information until they're ready for it -- which is shown by them asking about it. (And I mean asking in the sense of "Okay, tell me about that statuette" or similar, it doesn't need to be specific.)
This also helps keep your players engaged by making them active participants in describing the room. Have you ever run your players through an exploration section and started to feel like it's just you reading to the players while they occasionally go "I open the next door" or "We go down the hallway" without really involving themselves in the game? I sometimes feel like I'm doing Storytime when I'm describing a series of rooms or environments. It's a lot more interesting when they're asking me questions than for me to just blurt out line after line of room description with minimal interaction.
The animated rock
Did you ever watch an animated show and notice a wall or rock that's animated in a slightly different style or color than the surroundings, which lets you immediately identify that something is about to move or destroy that one piece of the scenery? I try to avoid the D&D equivalent of that, which is describing one object in much greater detail than everything around it, which makes it immediately obvious that this item is important. (Which may be true, or it might be you just accidentally made a big deal out of it, and now the players are fixated on something irrelevant!) Having a pattern to your descriptions can help avoid that, both by limiting your initial introductory description until the players ask about that item, and by giving you the opportunity to describe anything with extra details if the players are asking about it.
